It may be known that a rare-earth substance is extracted from a variety of source minerals such as monazite, bastnaesite, gadolinite and xenotime which contain a rare-earth concentrate in the forms of (Th,R)PO.sub.4, RFCO.sub.3, (Be,Fe)R.sub.2 Si.sub.2 O.sub.10 and RPO.sub.4, respectively. In refining, it is necessary therefore to treat each of these source materials or rare-earth concentrates to separate or free or rare-earth compound from its other components. This separation or dissociation step has heretofore been carried out purely chemically. Thus, monazite, bastnaesite or xenotime has been treated with sulfuric acid, sodium sulfate, sodium hydroxide, hydrogen chloride and/or ammonium chloride to yield the rare-earth compound in the form of R(OH).sub.3 or R.sub.2 O.sub.3 separated from thorium sulfate, hydrogen phosphate, sodium sulfate, carbon dioxide, silicon fluoride, hydrogen fluoride, sodium phosphate, sodium hydroxide, thorium hydroxide, thorium ions, sulfate radical and/or phosphate radical.
Not only do these prior dissociation processes require a high concentration of the reactant, H.sub.2 SO.sub.4, Na.sub.2 SO.sub.4, HCl, NH.sub.3 Cl, which adds to cost, but they also requires a relatively high temperature for the reaction which amounts to 200.degree. to 250.degree. C. or around 150.degree. C. to be employed and maintained for a relatively long time period amounting to several hours. Furthermore, a large amount of noxious gases such as SO.sub.3 and HF and even noxious alkaline mists may unavoidably be generated, necessitating a large and costly gas-treatment facility. It should be noted therefore that the prior dissociation processes are undesirable or unsatisfactory not only from energy and resource saving standpoints but from economical consideration for the equipment as well as cost consideration of rare-earth elements that are produced thereby.
Another problem encountered heretofore in the rare-earth material field is that one existing dissociation process (e.g. an alkaline dissociation process) applicable to one particular rare-earth mineral (e.g. monazite) is generally not applicable to another mineral (e.g. bastnaesite). Thus, where two different types of the rare-earth source are produced in a mixed mineral as, for example, produced by the Baiyun mine in China which contains 40% monazite and 60% bastnaesite, the prior-art technique has been prohibitively inefficient or even impractical.